An inkjet printer has conventionally been known as one example of a liquid ejection apparatus. The inkjet printer ejects ink as one example of a liquid from an ejection head onto a printing medium such as printing paper, so as to implement printing on the printing medium. A known configuration of the inkjet printer causes ink stored in a tank as one example of a liquid container to be supplied to the ejection head. This tank is provided with an ink inlet port. The user is allowed to refill ink from the ink inlet port into the tank (see, for example, JP 2012-51309A). In the description below, the configuration that the liquid container such as tank is added to the liquid ejection apparatus such as inkjet printer may be called liquid ejection system.